The present invention relates to a purge plug which is adapted to be replaceably attached to the bottom of a ladle for purging gas through a heat in the ladle, the purge plug comprising a ceramic body having at least one through gap which extends between the end faces of the body and a casing which encloses the body, the gas being fed to the external end face of the body in order to flow, at a predetermined pressure, through said gap to the internal end face of the body and into the heat.
Purge plugs are generally used for stirring a heat in a ladle, and, where appropriate, also for modifying the heat, and serve to let gas under high pressure, for instance 6-10 bar, into the ladle. A purge plug traditionally consists of a gas-permeable, ceramic cone having the shape of a truncated cone of about 10 kg which is enclosed by a sheet-metal casing. The purge plug is generally supplied in so-called sets, that is the cone is mounted in a hollow block which weighs about 50 kg.
The purge plug has a key role when manufacturing pure steel and is used by practically all steel plants in the world. One or more purge plugs are replaceably mounted in the bottom of a unit which is found in all steel plants and is called a ladle, that is a container lined with refractory material and adapted to contain melted steel. In the ladle, various high-temperature chemical processes are carried out in which the purge plug is crucial. Gas, generally argon, is purged through the purge plug. The purge plug is worn heavily during the process in its uppermost portion, which is directed towards the heat, and regularly has to be replaced by a new purge plug when its height has shrunk to a minimum permissible level.
The first purge plugs which were made were porous, that is they were gas-permeable but did not let steel through. The disadvantage of these purge plugs was that high pressure gas was required to obtain a sufficient flow, and owing to their porosity the wear was considerable.
The next step in the development of purge plugs was so-called directional porosity, that is a number of ducts with a diameter of about 0.5 mm were formed by casting and gas was purged through the ducts. The advantage of this purge plug was that it was easy to achieve a gas flow through the plug and that the plug could be made compact, which meant less wear. The disadvantage was a greater risk of infiltration of steel in the ducts, which obstructed the purge plug.
The third step in the development of purge plugs was to form slots or gaps by casting, having a thickness of about 0.2 mm and a length of about 20 mm, through the purge plug instead of ducts. The advantage of this was that it was easy to achieve a high gas flow, without infiltration of steel in the slots. However, there is always a risk of infiltration of steel as long as there are open gaps in the purge plug.
A purge plug must fulfil three requirements: a high degree of durability, satisfactory gas permeability and avoidance of infiltration of steel in the gaps of the purge plug.
Satisfactory durability is obtained by the purge plug being made of a high-quality, compact refractory material.
A high degree of gas permeability is obtained by the gaps having a sufficient sectional area.
Minimum infiltration is obtained by the gaps being sufficiently narrow.
These three requirements counteract one another and a purge plug is a compromise of these three properties.
German patent specification DE 196 10578 discloses a purge plug which comprises an external body having a truncated cone-shaped cavity in its upper portion, a cylindrical, enlarged cavity in an intermediate portion and a cylindrical duct in its lower portion. The cavities communicate with one another to allow gas with a specific pressure flow from one end face to another. In the cavities, in the above-mentioned order, a truncated cone, a piston and a tubular piston rod which is provided with openings are movably arranged as a unit. The cone is pretensioned to a closing position by means of an external pressure spring which engages with the free piston rod end protruding from the purge plug. When gas with a sufficient pressure (in order to counteract the force of the spring and the metallurgical pressure) is supplied through a gas supply pipe and via the piston rod to the enlarged cavity, the piston raises the cone and the gap between the cone-shaped cavity and the cone is opened. Then the gas can flow round the piston and through the gap into the heat.
The disadvantages of this construction are as follows:
The construction is very difficult to manufacture as purge plugs are generally cast in a refractory material. It is technically difficult to make an intermediate, enlarged cavity (16) by casting since the template used in casting is very difficult to remove.
The construction has complicated protruding parts (the spring suspension beneath the purge plug), which are inconvenient in a steelworks environment since they may easily break by being subjected to impacts or other rough handling.
The spring suspension is expensive to manufacture.
The purge plug is worn gradually. When it is worn down to the enlarged cavity (16), a breaking through of the plug is obtained and steel flows through the bottom of the ladle with disastrous results. It is thus very important that the plug not be worn down to this level. Advanced equipment is required in order to determine how far down the purge plug has been worn.
Another patent that is based on a purge plug having a movable body and a fixed body, where the gas flows through an annular gap, is U.S. Pat. No. 4,470,582.
What mainly makes this construction different from that in the above-mentioned German patent is that the pressure spring has been replaced by a lever mechanism which is preloaded by means of a hydraulic cylinder.
The disadvantages of this construction are as follows:
Because of its complicated shape, it is very expensive to manufacture.
The gas flow is controlled by an external valve which opens and closes the annular gap by guiding the movable body upwards and downwards, respectively. The disadvantage is that an external controlling device is required, which takes up space and is inconvenient in a steelworks environment. It can easily be damaged by external forces.
As in the above-mentioned German patent, the purge plug is worn with the same result as indicated above.
One object of the present invention is to provide a purge plug having a gap, the width of which is a function of the applied gas pressure and which is easy and inexpensive to manufacture.
Another object of the invention is to provide a purge plug having an adjustable gap which does not require complicated mechanisms and does not exhibit any protruding components.
Yet another object is to provide a reliable purge plug which can be worn down more or less completely before the breaking through takes place.
According to the invention these objects are achieved by a purge plug as mentioned by way of introduction, which is characterised in that the ceramic body comprises an external essentially sleeve-shaped body portion and an internal, truncated cone-shaped body portion between which said gap is arranged, that the internal body portion is movable in relation to the external body portion and is pretensioned by means of a resilient means in a direction towards the heat for closing the gap and that a gas supply pipe is connected to a space beneath the external end face in order to open the gap.
Further developments of the invention are evident from the features indicated in the subclaims.